This invention relates to optical coupling arrangements and, more particularly, to optical coupling arrangements suitable for use in high bandwidth, free-space optical communications systems operating at terra-Hertz (THz) signaling rates.
High-bandwidth transmitters and receivers (e.g.,  greater than 1 GHz) for signaling at rates above 650 MHz present a twofold challenge. First, stray capacitance must be minimized, which dictates that solid state detectors have a very small active area, e.g., an aperture of less than 25-30 xcexcm in diameter. This presents a significant challenge in receiver design since optical signals must be gathered, aligned with, and coupled into a very small aperture of the solid-state detector surface with minimal loss.
At the outset, it would seem to be quite straight forward to use a lens (or mirror) to focus a light beam down to any desired spot size. To minimize the size of the receiver it would be desirable to use a lens with a short focal length and to gather the largest column of incident light, it would be desirable to use a lens with the largest possible entrance pupil. This dictates that a lens with a small xe2x80x9cfxe2x80x9d number (the ratio of focal length to diameter of the entrance pupil) should be used. Because lens aberrations, atmospheric turbulence, misalignment, and manufacturing errors all cause the focused spotsize to be much larger than the diffraction-limited size, use of a smaller xe2x80x9cfxe2x80x9d number than that dictated solely by diffraction effects seems unavoidable. On the other hand, lenses with small xe2x80x9cfxe2x80x9d number are quite expensive and difficult to manufacture.
In our co-pending patent application entitled xe2x80x9cHigh Speed Optical Receiverxe2x80x9d, Ser. No. 09/702252, filed Oct. 30, 2000, the substance of which is hereby incorporated by reference, we described a method of increasing the effective area of a high-speed detector by affixing a non-imaging concentrator element to the face of the high-speed detector. In particular, a compound parabolic concentrator (CPC) was used to channel light from a large entrance aperture to the necessarily smaller exit aperture near the detector face. A somewhat similar approach is disclosed by Ruben Mohedano, Juan C. Minano, Pablo Banitez, Jose, L. Alvarez, Maikel Hernandez, Juan C. Gonzalez, Kazutoshi Hirohashi, and Satoru Toguchi, in an article entitled xe2x80x9cUltracompact non-imaging devices for optical wireless communicationsxe2x80x9d Opt. Eng. 39(10), pp. 2740-2747, (October 2000).
The aforementioned application and journal article did not discuss the role of rays exiting the CPC at extreme angles, e.g., approaching 90xc2x0 to its optical axis. Although the concentration ratio achieved by the CPC would appear to approach the ideal limit for such rays, such rays would be parallel to the detector surface and could neither be absorbed nor detected. Moreover, light striking the detector surface at or near grazing incidence would simply be reflected at the detector surface and would also not be detected. Because a significant distribution of light rays exit the CPC aperture at angles approaching 90xc2x0, the detection efficiency of the CPC/detector combination is less than might be expected.
One approach to increasing the detection efficiency of the CPC/detector combination might lie in providing an antireflective (AR) coating on the surface of the detector so that rays approaching grazing incidence would not be reflected. While we do not reject this approach, as a rule-of-thumb, the best available AR coating materials have a cut-off angle of about 30xc2x0 to 40xc2x0, depending on the substrate material, and their use would not result in the achievement of maximum detection efficiency. It would be extremely advantageous to be able improve the coupling efficiency of the CPC/detector combination to permit the detection of rays incident upon the detector at and beyond the grazing angle of the uncoated detector surface.
In accordance with an aspect of the present invention, the coupling efficiency of a CPC/detector is increased by treating the surface of the detector so that incident light can be detected at or near the maximum angle of light gathered by the CPC. The detector surface is treated so that it presents an aperiodically distributed array of microscopic silicon spikes, each spike having a quasi-conical aspect. In one embodiment, the spikes exhibit conic angles of approximately 20xc2x0 on average, so that the overall xe2x80x9cblaze anglexe2x80x9d of the micro-structured detector surface is such that light is channeled into rather than out of the detector surface. When provided with such a micro-structured surface, nearly all of the incident light at nearly any angle of incidence are absorbed into the detector. The micro-structured surface may be effected by bombardment of the silicon substrate of the detector with high-intensity ultrafast laser pulses. Alternatively, the micro-structured surface can be formed on n-type (arsenic doped) silicon, independent of dopant concentration so that the micro-structured surface may be generated on the first layer of a semiconductor optical detector diode. Furthermore, the microstructures may, with equally advantageous results, be formed on silicon having other dopants (e.g., p-type), and even on other semiconductor materials that may be desired in the front layers of high-speed optical detectors selected for use with non-imaging light gathering concentrators.